Inertial sensors, such as accelerometers, have wide applications in many industries. Most notable perhaps being in the aerospace, military, and automotive industries. More recently, they may be found in computer video game controllers where the controller senses user body movements.
One type of traditional accelerometer is the mercury switch. Typically this comprises a sealed tube containing a pair of electrodes and a small amount of mercury. When the tube is tilted or the mercury otherwise accelerated it makes contact with the electrodes and completes an electrical circuit. This may be considered a type of one-bit accelerometer; one bit, because it's either on or off. Unfortunately, mercury is toxic and containment may be an issue. Further, such switches are relatively large, and cannot be fabricated by photolithography.
Accelerometers may be used to measure acceleration, vibration, and mechanical shock etc. Single-axis, dual-axis, and triple-axis accelerometers are available to measure acceleration as a vector quantity in one or more dimensions. Modern accelerometers may be fabricated as micro electro-mechanical system (MEMS) devices. MEMS accelerometers typically comprise a suspended cantilever beam or proof mass with some type of deflection sensing circuitry. As forces cause the accelerometer to accelerate/decelerate, inertia may cause the cantilever or proof mass to deflect relative to the frame or supporting structure of the rest of the device. The deflection quantity and direction may be sensed and measured to provide an acceleration vector.
Referring now to FIG. 1, there is shown a piezoelectric MEMS sensor which may be used to sense acceleration or vibration by converting mechanical energy into an electrical signal. The sensor may include a generally rigid frame 100 or support structure. Here the frame 100 is generally square, but other shapes are possible. A proof mass 102 is suspended at the center of the frame 100 by a plurality of arms 104 attached at the corners of the frame 100. A beam 106 surrounds the proof mass 102 within the frame and defines two capacitors 108 and 110 for each arm 104. Thus, the piezoelectric sensor shown in FIG. 1 comprises eight capacitors on chip.
As shown in FIG. 2, this arrangement forms a 2-port system with outputs Q1 and Q2. Capacitors on adjacent arms are 90° degrees apart from each other, such that out-off-axis components (caused by accelerations in x or y direction) are cancelled assuming all four capacitors (C1=C2=C3=C4 and C5=C6=C7=C8) are the same.
The above design may have several drawbacks. First, depending on the selected piezoelectric material, the piezoelectric sensitivity changes which results in a limited/reduced dynamic range. Thus, it may be beneficial to increase the overall signal-to-noise ratio (SNR) of the piezoelectric MEMS sensors.
Second, as shown in FIG. 2, the state-of-the-art sensor has two signal ports, Q1 and Q2, plus ground. However, for a high performance sensor, a fully differential sensing and feedback structure would be desirable.
Finally, any mismatch due to the processing causes C1≠C2≠C3≠ . . . ≠C8. As a result one obtains a charge difference/imbalance (|Q1|≠|Q2|) resulting in an offset voltage at the output of the succeeding charge-to-voltage converter. In addition, the sensor gets sensitive to out-off-axis accelerations since these portions did not cancel due to ΔCx and ΔCy.